In the interest of more exacting control of the emissions of combustion engines, it has been suggested by many that greater control be exercised over the air-fuel ratio supplied to the engine; and one suggested manner of control is closed loop control using an oxygen sensor in the engine exhaust means. In particular, closed loop control about a predetermined ratio near stoichiometry allows the use of the so-called three way catalyst which simultaneously oxidizes hydrocarbons and carbon monoxides and reduces oxides of nitrogen. However, most such systems suggested have required replacement or modification of the carburetor with actuation means responsive to the feedback signals for controlling air-fuel ratio.